As is generally known, in recent years there have been proposed and developed various variable valve actuation devices, in which a working angle of an engine valve (an intake valve and/or an exhaust valve) can be variably controlled depending on an engine operating condition, in order to ensure improved fuel economy and stable driveability (improved operational stability of the engine or stable engine speeds) during low-speed and low-load operation and also to ensure a sufficient engine power output caused by an enhanced intake-air charging efficiency during high-speed and high-load operation. One such variable valve actuation device has been disclosed in Japanese Patent Provisional Publication No. 2002-256832 (hereinafter is referred to as “JP2002-256832”), corresponding to U.S. Pat. No. 6,550,437, issued on Apr. 22, 2003 and assigned to the assignee of the present invention. The variable valve actuation device disclosed in JP2002-256832, often called “continuous variable valve event and lift control (VEL) system”, is configured to adjust an angle of oscillation of a rockable cam by varying a fulcrum of oscillating motion of a rocker arm by rotary motion of a control cam attached to the outer periphery of a control shaft, thereby variably controlling a valve lift and a working angle of an intake valve. Additionally, the variable valve actuation device disclosed in JP2002-256832 has an intake-valve lift characteristic that a maximum positive acceleration of the valve lift increasing side (that is, a maximum positive valve opening acceleration) is set to be greater than a maximum positive acceleration of the valve lift decreasing side (that is, a maximum positive valve closing acceleration), regardless of the magnitude of working angle of the intake valve. In other words, regarding each and every working angle of the intake valve, the variable valve actuation device of JP2002-256832 has a so-called “forwardly-inclined unsymmetrical valve lift characteristic” in which the intake-valve lift characteristic curve is slightly inclined forwards (see FIG. 3A of U.S. Pat. No. 6,550,437). For instance when the intake valve has been controlled to a maximum working angle in a high-speed range, by virtue of such a “forwardly-inclined unsymmetrical valve lift characteristic”, it is possible to suppress an abnormally re-seating behavior (that is, “jumping” or “valve bounce” phenomenon) of the intake valve in the last stage of valve-closing motion, thus suppressing an excessive load (e.g., an impact load) from being undesirably applied to the valve actuation mechanism, in other words, an undesirable deformation of the valve actuation mechanism.